feat(rapier_testbed): autosave testbed configuration + add support for per-example settings (#799)

* feat(rapier_testbed): autosave testbed configuration + add support for per-example parameters

* feat(rapier_testbed): also save the camera position

* feat(rapier_testbed): improve ergonomics of example-specific settings

* chore: cargo fmt

* chore(rapier_testbed): small UI ergonomics improvement

* chore(rapier_testbed): clippy fixes

src_testbed/lib.rs

@@ -23,6 +23,8 @@ pub mod physics;
 #[cfg(all(feature = "dim3", feature = "other-backends"))]
 mod physx_backend;
 mod plugin;
+mod save;
+mod settings;
 mod testbed;
 mod ui;